The present invention relates to adjustment mechanisms for adjusting motor vehicle seats, and to seats equipped with such mechanisms.
More particularly, the invention relates to an adjustment mechanism for adjusting a motor vehicle seat, said adjustment mechanism comprising a worm screw mounted in a nut which is carried by a rigid base, the worm screw being mounted to rotate relative to the base to cause the screw and the base to move in translation relative to each other along a longitudinal axis, the nut normally being held in a rest position relative to the base, and said nut being mounted on the base with a certain amount of clearance along the longitudinal axis.
Document FR-A-2 796 013 describes an example of such an adjustment mechanism, which is quite satisfactory.
A particular object of the present invention is to improve adjustment mechanisms of that type still further, in particular so as to increase their strength and/or to enable lower-strength nuts to be used without penalizing the strength of the mechanism as a whole.
To this end, in the invention, an adjustment mechanism of the type in question further comprises at least one locking piece which is mounted to move relative to the base between firstly a retracted position in which said locking piece does not interfere with the worm screw and secondly at least a first active position in which said locking piece comes into engagement with the worm screw to prevent said worm screw from moving relative to the base;
the nut is adapted to act on the locking piece when said nut is moved at least in a first direction along the longitudinal axis from the rest position of said nut;
and the locking piece is mounted on the base such that the nut moving in said first direction causes said locking piece to move towards its first active position.
By means of these provisions, when the worm screw is subjected to a large force along the longitudinal axis, it is prevented from moving relative to the base not only by the nut, but also by the locking piece, thereby making the adjustment mechanism very strong in the event of an accident.
Thus, the nut can be dimensioned to take up only the normal forces transmitted by the worm screw during normal use, which makes it possible, for example, to use a nut that is relatively small, hence:
compactness is improved; and
friction is reduced, and thus wear on the screw is reduced, and noise during use is reduced.
These provisions also make it possible to use a nut made of a material that is not as strong as steel, e.g. of a plastics material that is not filled with glass fibers or with carbon fibers, which makes it possible to achieve quiet operation and low wear on the worm screw, compared with a nut made of steel or with a nut made of a plastics material filled with glass fibers or with carbon fibers.
Naturally, the invention is not limited to such nuts made of a plastics material, but rather it can be used regardless of the material of the nut, including a metal material.
In preferred embodiments of the invention, any of the following provisions may optionally be used:
the nut is made of a plastics material not filled with reinforcing fibers;
the nut is adapted to push the locking piece at least in the first direction, and said locking piece has a first abutment surface in contact with a first cam surface belonging to the base, said first abutment surface and said first cam surface being adapted to move the locking piece towards the worm screw when said locking piece is moved by the nut in said first direction;
the first cam surface is in the shape of a ramp converging towards the worm screw in the first direction, and the first abutment surface of the locking piece is also in the shape of a ramp that is substantially parallel to the first cam surface;
the worm screw is provided with at least one helical thread and the locking piece is provided with a first set of teeth adapted to engage in said thread when said locking piece is in its first active position;
the base has a rigid cage which surrounds the worm screw and which has two abutment faces disposed on either side of the nut along the longitudinal axis, two flexible buffers being interposed between the nut and respective ones of said abutment faces to urge the nut resiliently into its rest position;
the nut is made of a plastics material, and is adapted to deform plastically against said abutment face when the worm screw is subjected to a force that is larger than a limit value along the longitudinal axis, and the locking piece is adapted to press the worm screw against the cage when the nut deforms plastically against said abutment face;
the adjustment mechanism further comprises at least one runner designed to enable a motor vehicle seat to be adjusted longitudinally, said runner comprising first and second rails which extend along the longitudinal axis, the first rail being secured to the base, and the worm screw being mounted to rotate on the second rail; and
the nut is further adapted to push the locking piece in a second direction opposite from the first direction, and said locking piece further has a second abutment surface in contact with a second cam surface belonging to the base, said second abutment surface and said second cam surface being adapted to move the locking piece towards a second active position in which said locking piece comes into engagement with the worm screw when said locking piece is moved by the nut in said second direction; the second cam surface is in the shape of a ramp converging towards the worm screw in the second direction and the second abutment surface of the locking piece is also in the shape of a ramp that is substantially parallel to the second cam surface; the locking piece is provided with a second set of teeth adapted to engage in said thread when said locking piece is in its second active position; and, between its first and second sets of teeth, the locking piece has a notch open towards the worm screw and in which a tab belonging to the nut is engaged, with a certain amount of clearance making it possible for the locking piece to move towards the worm screw from its retracted position.
The invention also provides a vehicle seat having a seat proper whose longitudinal position can be adjusted by means of an adjustment mechanism as defined above.